Among others, for testing communication systems and communication equipment, such as xDSL transceivers and cables or networks, test signals are needed for stressing the communication system and the communication devices in a manner that is representative to actual deployment scenarios, with large numbers of systems or system devices per cable.
By measuring the transmission performance of the system or system device under realistic (noisy) test conditions, one can improve the design of the system or devices and/or prove that their performance is compliant with standards, such as issued by ETSI, ITU or ANSI or other (tele)communication bodies.
A method of executing such performance tests is to generate a signal which is known as impairment. More specifically, impairment can be subdivided into:                (i) cross-talk noise, having a noise profile characterized by a spectral envelope and spectral amplitude distribution e.g. from neighboring xDSL systems;        (ii) ingress noise, composed of discrete frequency components, also called rfi-tones, having a noise profile characterized by a number of discrete frequency components and spectral amplitude, modulation depth and modulation width parameters originating from radio and amateur broadcasting, for example, and        (iii) impulse noise characterized by signal pulses caused by switching operations and components for example.        
In the case of ingress noise, the frequency may vary (sweep) in time.
A device for generating impairment is known as an impairment generator and is arranged, in particular for use in or on communication systems, for generating at least one of said cross-talk noise and ingress noise.
In practice, for testing whether communication systems and communication devices are compliant with standards, various noise profiles have been defined which, among others, vary in accordance with system parameters such as the length and number of wire pairs in a communication cable and the transmission data rate, for example.
Further, each different type or length of a transmission medium such as a cable, a copper cable or an optical fiber or other cable type, request a different noise signal.
Methods and devices for generating noise profiles are known in the art. In particular, filtering techniques and filters are known for generating noise from an input signal providing an output signal having a particular spectral envelope and spectral amplitude distribution.
However, by using filtering techniques and filters, a causal relationship is established between the input signal and the output signal. Those skilled in the art will appreciate that such a type of signal is less suitable for a realistic imitation of real operational communication systems and communication devices.
WO 00/16181 discloses a method and a device for generating a random time domain signal approaching a predetermined histogram of amplitudes. In a first step, the signal is created by filtering a noise signal, such as a white noise signal, thereby producing a signal having a predetermined spectral envelope. In a next step, a non-linear function is applied to the filtered noise signal, so as to produce the required time domain signal approaching the predetermined histogram of amplitudes. In a further step, pulse response filtering is applied to the time domain signal, to correct its spectral envelope and to obtain an output signal having a required spectral envelope. Both, the non-linear function and the pulse response filtering function are special functions selected in accordance with the spectral envelope to be provided.
WO 00/16181 is limited in the sense that there is only provided a time domain signal only having a predetermined spectral envelope. WO 00/16181 is silent with respect to other quality criterion's to be imposed on the time domain signal to be provided, among others phase properties.